Concrete blocks, often referred to as concrete masonry units (CMU's), are typically manufactured by forming them into various shapes as part of an automated process employing a concrete block machine. Such concrete block machines employ a mold having one or more cavities in which a block is formed, with each cavity having a shape of the block desired to be formed. The mold is bolted onto/into the concrete block machine and has an open top and an open bottom.
During a block forming process, a pallet is moved by a conveyor system onto a pallet table which, in turn, is moved upward until the pallet contacts the mold and forms a bottom for each of the one or more mold cavities. A feedbox filled with dry cast concrete is then moved from a retracted or withdrawn position to an extended position above the mold frame where it fills the one or more mold cavities with dry cast concrete via the open top. A cutoff bar which is fixed-mounted to the feedbox assembly scrapes or wipes away excess dry cast concrete from the top of the mold cavities as the feedbox is driven back to the retracted position. The block machine then moves a head shoe into the mold cavities via their open tops and compresses the dry cast concrete to a desired psi (pounds-per-square-inch) rating while simultaneously vibrating head shoe, mold cavity, pallet, and pallet table.
As a result of the compression and vibration, the dry cast concrete reaches a level of “hardness” which enables the resulting molded blocks to be immediately removed from the mold cavities. To remove the molded blocks from the cavities, the mold remains stationary while the head shoe, pallet, and pallet table move downward and force the molded blocks from the mold cavities. The conveyor system then moves the pallet bearing the molded blocks away to be cured and a clean pallet takes its place. This process is continuously repeated in an automated fashion to produce additional blocks.
For many types of CMUs (e.g. pavers, patio blocks, light-weight blocks, cinder blocks, etc.), retaining wall blocks and architectural units in particular, it is desirable for at least one surface of the block to have a desired texture, such as a stone-like texture, for instance. When arranged to form a structure with the textured surface being visible, the structure will have the appearance of being constructed from natural stone, for example.
One technique for creating a desired texture on a block surface is to provide a negative of a desired texture or pattern on a moveable side wall of a mold cavity. During the block forming process, the moveable side wall is moved to an extended position to form the mold cavity. As described above, the mold cavity is then filled with dry cast concrete and compressed/vibrated. The moveable side wall is then moved to a retracted position and the molded block having the textures surface is removed from the mold cavity for curing, as described above. Textured block surface can also be formed by shearing or splitting off a block face as the molded block is removed from the mold cavity through use fixed studs extending from and forming a texture of sorts on a corresponding side wall of the mold cavity.
While such techniques are effective at forming textured surface on the molded blocks, air pockets trapped between the textured surface of the side walls of the mold cavity and dry cast concrete filling the mold cavity are forced out during the compression/vibration process, causing the concrete to settle along the side wall of the mold cavity forming the textured block surface. As a result, the textured surface of the block may not be completely formed and the molded block may have a height along the textured surface (e.g. front face of block) which is shorter than that along an opposite surface (e.g. rear face of block).
To compensate for the settling of the dry cast concrete, the fixed cutoff bar is sometimes made to be narrower along its edges than at its middle. As a result, as the feedbox is moved to its retracted position and the cutoff bar is drawn across the top of the concrete-filled mold cavity, more dry cast concrete is left along the edges of the mold cavity which are parallel to the direction of travel of the feedbox than in the middle of the mold and along edges which are perpendicular to direction of feedbox travel. While such a technique is generally successful at providing more concrete for a textured surface when the textured surface of the block is located along edges of the mold cavity parallel to the direction of travel of the feedbox, it does not work when the textured side walls (e.g. the moveable side walls) and thus the textured surface of the molded block are along edges of the mold cavity which are perpendicular to the direction of travel of the feedbox.